1. Field of the Invention
The invention relates to an apparatus and a method for determining the telecentricity of an optical imaging system, and to a microlithography projection exposure apparatus which is provided with such an apparatus.
2. Field of the Related Art
As is known, the determination of telecentricity is used to identify deviations from the ideal telecentric behavior of optical imaging systems, i.e. telecentricity errors. In an imaging system which is affected by a telecentricity error, the primary ray for a respective field point travels not parallel to the optical axis of the imaging system as in the error-free case, but tilted relative thereto, the tilt angle representing a quantitative measure of the telecentricity error. This per se suggests that the telecentricity error can be determined by measuring the energy centroid position of the image of a respective field point in an xy plane perpendicular to the optical axis, at a plurality of measurement points which are mutually offset in the z direction of the optical axis, and by calculating the tilt angle trigonometrically therefrom. This, however, is faced with the difficulty that the energy centroid position of the image of a respective field point in the xy plane may also vary as a function of the z position because of other image errors which typically affect imaging systems, such as coma and Petzval correction errors. In general, therefore, the telecentricity error cannot be deduced merely from such a measurement of the energy centroid position in the xy plane as a function of the z position.
For example, it is conceivable to determine the telecentricity by a Moiré technique in which a first Moiré structure, arranged in an object plane of the imaging system, is projected onto a second Moiré structure, identical except for the imaging scale, arranged in an image plane of the imaging system. This creates a Moiré superposition pattern in the image plane, which, as is known, exhibits so-called Moiré fringes when the imaging system is affected by a distortion error. If the z position of the Moiré structure on the image side is subsequently modified, the Moiré superposition pattern will not change in the ideal case of an imaging system which is free from aberration and is telecentric on the image side. Yet if the imaging system has a telecentricity error but is otherwise free from aberration, then the Moiré superposition pattern changes and the telecentricity error can then be found in this case from its variation as a function of the z position. If the imaging system is also affected by aberrations, however, then variations in the Moiré superposition pattern due to this are superimposed on those which are caused by the telecentricity error. If illumination which does not fully and uniformly fill a pupil of the imaging system is selected for the Moiré measurement, then this entails another perturbing effect when trying to find the telecentricity error.
Further perturbing effects may result from planarity errors of a measurement reticle placed on the object side, or a measurement surface placed on the image side, since with such a so-called staggered focus measurement, i.e. a measurement at different z positions, a different region of the air image is respectively recorded for various field points and employed for calculating the telecentricity error.